The electricity generation by thermoelectricity is a technique utilizing Seebeck effects in semiconductor materials for realizing direct conversion from heat to electricity, which is characterized in long lifespan, high reliability, safe environment, etc. It has wide applications and potential social and economic effects in areas of electricity generation by photoelectricity and thermoelectricity solar energy and electricity generation by industrial waste heat. Improving the figure of merit of thermoelectric materials is the key point to improve energy conversion efficiency of thermoelectric electricity generation. Therefore, studies in the field of thermoelectric conversion mainly focus on developing new thermoelectric materials with high performance. In another aspect, process of researching and developing new thermoelectric material devices is of equal importance for improving energy conversion efficiency of thermoelectric electricity generation.
Thermoelectric devices mainly comprise two types of thermoelectric semiconductor components, p type and n type. Since the voltage of a single thermoelectric device is very low, electrodes are usually used to have a variety of p type and n type thermoelectric components connected in series for electric conduction or connected in parallel for thermal conduction to construct a thermoelectric electricity generation module, thereby to acquire a higher voltage for usage.
Filled skutterudite is regarded as a new thermoelectric material in an intermediate temperature with high performance, which has a promising future of application. A technique for welding Bi2Te3 device is borrowed for welding electrode in a low temperature of a filled skutterudite device, where copper is chosen as the electrode material and the technique of tin soldering is adopted for welding. In respect of welding electrodes in a high temperature of filled skutterudite devices, according to existing reports, Cu, Mo, Ni—Cr, W, Ta, and their alloys, stainless steel (U.S. Pat. No. 6,005,182), Ag, Ag—Au, Ag—Cu, Fe (U.S. Pat. No. 6,759,586) and Nb (U.S. Pat. No. 6,563,039) are chosen as electrode materials, while copper brazing (U.S. Pat. No. 6,005,182, US2002/0024154, CN101114692, etc.), silver brazing (U.S. Pat. No. 6,759,586, US2008/0023057, etc.), sintering (US2006/0017170, U.S. Pat. No. 6,563,039, JP11195817, etc.) and the like are adopted as joining methods for electrodes and skutterudite materials.
Table 1 lists thermal expansion coefficients (CTEs), electrical conductivities and thermal conductivities of skutterudite and metallic materials. It can be seen that simple metals, except Ti, Fe, and Ni that have a CTE close to that of filled skutterudite, show large differences in CTE than filled skutterudite, while Ti, Fe and Ni, exhibit much lower electrical conductivity and thermal conductivity than Cu, Mo, etc. Stainless steel, which mainly composes of Fe, Cr, Ni, etc., has a CTE closest to that of the filled skutterudite material. In addition, it is observed that Mo has a smaller CTE than the filled skutterudite material while Cu has a larger CTE than filled skutterudite. When Mo and Cu are combined into alloy, the alloys may have CTE close to that of filled skutterudite material by adjusting relative proportion of the two, and may also maintain good electrical conductivity and thermal conductivity of Cu and Mo. W and Cu are quite the same.
Recently, a commonly adopted method for fabricating thermoelectric devices (for example, the method for fabricating thermoelectric devices recorded in a CN invention patent application NO. 200710044771.0) is mainly characterized in steps of: first fabricating (sintering) a bulk element of a thermoelectric device in a die, welding an electrode at a high temperature onto the bulk element, welding the side at a low temperature with a ceramic plate by solders, and then forming a π shape thermoelectric device by cutting or the like eventually. However, the existing method is not only complex, but also inevitably to expose thermoelectric materials (such as filled skutterudite) again to heat and pressure with a risk of degrading the performance of thermoelectric materials. Therefore, it is in urgent need of developing a new method for fabricating devices to simplify processing steps and avoid adverse impacts on thermoelectric materials.
TABLE 1CTE, electrical conductivity and thermal conductivity of materialselectricalthermalCTE (×10−6 K−1)conductivityconductivityMaterials(RT~875K)(×106Ω−1 m−1)(W/mK)CoSb310~11P: 0.062~0.073,P: 2.1~2.6,basedN: 0.11~0.23N: 2.2~3.0skutte-(RT~850K)(RT~850K)ruditeMo5.6~6.218.1138Cu18  59.6334W4.5~4.6(RT~100)18.9138Ti8.4~8.6(RT~100)1.921Ni13(RT~100)1682.8Fe12~13(RT~100)~437Ag19(RT~100)62.9429Ta6.5(RT~100)8.0357.5Nb7.2~7.3(RT~100)853.7stainless10~13(RT~100)1.5-2.514-16steelMo50Cu5010.5~9.5 37.1230~270Mo70Cu308.9~8.522.3170~200WCu alloy9.054.3220~230